Heat-insulating refractory material

ABSTRACT

The present invention is intended to provide a heat-insulating refractory material, which does not contain environmentally and hygienically undesirable ceramic heat-insulating fibers, but only contains organic fibers to the extent necessary to improve adhesive rate upon spraying, and which can nevertheless achieve a specified heat-insulating effect. The heat-insulating refractory material includes a mixture containing 2-50 wt % of a foaming raw material foamed by heat at a temperature from 400° to 1500° C. and 50-98 wt % of a refractory powder, and a liquid binder which is added to the mixture in an amount of 20-250 wt % on the basis of the weight of the mixture.

TECHNICAL FIELD

The present invention relates to a heat-insulating refractory materialfor suppressing heat dissipation, which is used for the surfaces ofrefractories of linings in an industrial furnace or the surfaces ofrefractories for steel casting.

BACKGROUND ART

As refractories for steel casting, into which molten steel at about1500° C. is poured, there has been used a graphite containing refractorycomposition of high thermal shock resistance. The refractory compositionhas a high thermal conductivity because of the contained graphite, andit allows a large amount of heat dissipation from the outer surfacethereof. The graphite containing refractory composition is pre-heatedbefore being used to prevent the generation of cracks due to abruptthermal change; however, these prevention abilities are limited becausethe temperature is rapidly dropped for a period of time from thecompletion of heating to the beginning of casting.

To cope with the above problem, for example, Japanese Unexamined UtilityModel Publications Nos. SHO 55-43616 and SHO 60-34354 disclose a methodof attaching Al₂ O₃ --SiO₂ or Al₂ O₃ --SiO₂ --CaO based ceramicheat-insulating fibers on the surfaces of refractories. This method,however, is disadvantageous in that while the contained fibers providehigh heat resistance and heat-insulation, the fibers, which have sizesin the order of several microns and are harmful to the human body, areset loose in the atmosphere when the heat causes loss of the binder.

As a means for obtaining a general heat-insulating structure in place ofthe above ceramic heat-insulating fibers, a monolithic refractorymaterial mixed with organic fibers lost by heating may be proposed.However, to obtain a good heat-insulating effect with only organicfibers, a large amount of the organic fibers must be added, which hasthe disadvantage of significantly reducing the strength of therefractory material.

The subject to be solved by the present invention is to provide aheat-insulating refractory material, which does not containenvironmentally and hygienically problematic ceramic heat-insulatingfibers, only contains organic fibers to the extent necessary to improveadhesive rate upon spraying, and can nevertheless achieve a specifiedheat-insulating effect.

DISCLOSURE OF THE INVENTION

The present invention has been accomplished under the basic premise thatit becomes possible to achieve an excellent heat-insulating effectwithout any of the above disadvantages by coating a foaming material ofhigh heat-insulating effect on the refractory surfaces of linings of anindustrial furnace or the surfaces of refractories for steel casting,and then foaming the coated material by heating or by the heat of moltensteel.

Namely, the present invention includes a heat-insulating refractorymaterial, which is composed of a mixture containing 2-50 wt % of afoaming material foamed by heat at 400°-1500° C. and 50-98 wt % of arefractory powder, a liquid binder which is added to the mixture in anamount of 20-250 wt % on the basis of the weight of the mixture.

A glass powder softened and molten at 400°-1500° C. may be further addedin an amount of 0.1-100 wt % on the basis of the above mixture; ororganic fibers lost by heating, such as yarns, pulp cotton, or vinylonfibers may be further added in an amount of 0.01-5 wt % on the basis ofthe above mixture to enhance the adhesive rate upon spraying.

The foaming raw material may include shirasu, vermiculite, obsidian,pearlite, pitch stone, expanded shale, fly ash and the like. Inparticular, obsidian or pearlite is desirable to generate a number ofbubbles by the addition of a small amount. The added amount of thefoaming material is required to be in the range from 2 to 50 wt %. Whenit is less than 2 wt %, the foamed amount is insufficient to achieve thedesired heat-insulating effect. When it is more than 50 wt %, theexcessive foamed amount imperils the strength required for theheat-insulating refractory material.

The glass powder is used for holding the gas generated from the rawmaterial foamed by heating within the coating film. When the refractorypowder becomes molten, the glass powder is not necessarily required.However, by the addition of the glass powder, the molten glass filmcovers the portion around the gas generated from the foaming rawmaterial particularly under the heating condition of 400°-1500° C. andprevents the gas from escaping outside the coating film. The glasspowder must be softened and molten at 400°-1500° C. to effectively holdthe gas generated from the foaming raw material at 400°-1500° C. withinthe coating film. When too much glass powder is added, the viscosity ofthe molten coating film is reduced and a continuous heat-insulatinglayer is not formed by the down-flow phenomenon of the coating film.Accordingly, the upper limit of the added amount is specified at 100 wt%.

As for the refractory powder, powders of roseki, silica stone, chamotte,mullite, alumina, fused silica, zirconia and the like may be used. Theadded amount is specified to be in the range from 50 to 98 wt %. Whenthe refractory powder added is less than 50 wt%, the fire resistance ofthe coating film is reduced, and a down-flow phenomenon of the coatingfilm molten at a high temperature is generated because of the reducedviscosity. When the refractory powder added is more than 98 wt %, theadded amount of the foaming raw material is relatively lowered, thusfailing to obtain a sufficient heat-insulating effect.

The organic fibers lost by heating are used to enhance the adhesive rateupon spraying, and are not necessarily required for the pouringoperation and brush-coating operation. When the added amount becomesexcessive, the strength of the heat-insulating refractory material issignificantly reduced when the fibers are lost by heating, and theadhesive rate is not significantly improved. Accordingly, the upperlimit of the added amount is specified at 5 wt %.

The foaming raw material, glass powder and refractory powder areadjusted in particle size so that the total particle size preferablybecomes 3 mm or less, and are kneaded with a liquid binder. Here, theliquid binder means a liquid material such as water, organic solvent,organic paste, silica sol, alumina sol or zirconia sol; and a solutionin which a powder binder is dissolved with water, organic solvent or thelike. The added amount of the liquid binder is specified to be in therange from 20 to 250 wt % to adjust the consistency of the foaming rawmaterial, glass powder and refractory powder so that they can be coatedon the surfaces of the furnace walls or graphite containing refractoriesby spraying, pouring or brushing. In the present invention, either ofthe above-described materials can be used; however, silica sol, aluminasol and zirconia sol are desirable to hold the coating strength at ahigh temperature.

The kneading is performed by a mixer in the usual manner.

The mixture thus kneaded is coated on refractories by pouring, sprayingor brushing, thereby forming a heat-insulating layer, which is expandedby the heat of molten steel or the like and forms blow-holes ofexcellent heat-insulation, over the whole surfaces of the refractories.

The foaming raw material contained in the heat-insulating refractorymaterial of the present invention is made to be in a slurried statetogether with the refractory powder and liquid binder. The compositionis foamed by heating before or after coating on the surfaces ofrefractories, and forms blow-holes being effective for heat-insulationover the whole surface. Moreover, by the addition of the glass powder,the gas generated from the foaming raw material can be effectively heldwithin the coating film.

According to the present invention, there can be obtained aheat-insulating refractory material, which does not containenvironmentally and hygienically undesirable ceramic heat-insulatingfibers, but only contains organic fibers to the extent necessary toimprove adhesive rate upon spraying, and which can nevertheless achievea specified heat-insulating effect. Moreover, the heat-insulatingrefractory material has a molten glass film containing independentbubbles, and therefore, it provides excellent shielding effect for air,and has the effect of preventing the oxidization of graphite containingrefractories.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a vertical sectional view of a testing apparatus for ensuringthe effect of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Unfoamed pearlite having an average particle size of 200 μm andvermiculite previously foamed by heating were used as a foamingmaterial; boro-silicate glass having a softening point of 800° C. wasused as a glass powder; roseki having an average particle size of 100 μmwas used as a refractory powder; and silica sol was used as a liquidbinder. These pearlite, vermiculite, boro-silicate glass and roseki wererespectively measured in weight of specified amounts, and then mixed.The silica sol was added to the mixture to such an amount (wt % based onthe weight of the mixture) as to ensure a consistency suitable forbrush-coating, and kneaded to be in the slurried state. The slurry wascoated onto the surfaces of a graphite containing refractories bybrushing, to form a heat-insulation layer having a thickness of about 3mm. This was put in a gas-furnace and heated at a temperature risingrate of 600° C./hr, and kept at 1100° C. for 2 hr, then naturallycooled, and taken out of the furnace. The refractory material was thenobserved for the formation state of the hollow structure using areflecting microscope.

                                      TABLE 1                                     __________________________________________________________________________             comparative example                                                                             present invention                                  sample No.                                                                             1     2     3     4    5    6    7    8    9                         __________________________________________________________________________    pearlite (%)                                                                           1     51    10    2    50   30   10   40   30                        roseki (%)                                                                             99    49    90    98   50   70   90   50   50                        vermiculite (%)                                                                        --    --    --    --   --   --   --   10   20                        boro-silicate                                                                          --    --    +101  --   --   +50  +100 --   --                        glass[softening                                                               point: 800° C.]                                                        (%)                                                                           silica sol (%)                                                                         +80   +80   +160  +80  +80  +120 +160 +100 +120                      state after                                                                            The   The   The   The  The  The  The  The  The                       heating at                                                                             hollow                                                                              hollow                                                                              coagu-                                                                              continu-                                                                           continu-                                                                           continu-                                                                           continu-                                                                           continu-                                                                           continu-                  1100° C. in a gas                                                               structure                                                                           structure                                                                           lated ous  ous  ous  ous  ous  ous                       furnace  could not                                                                           was   hollow                                                                              hollow                                                                             hollow                                                                             hollow                                                                             hollow                                                                             hollow                                                                             hollow                             obtained                                                                            broken                                                                              structure                                                                           structure                                                                          structure                                                                          structure                                                                          structure                                                                          structure                                                                          structure                          because                                                                             because                                                                             could be                                                                            could be                                                                           could be                                                                           could be                                                                           could be                                                                           could be                                                                           could be                           the   the   obtained                                                                            obtained.                                                                          obtained.                                                                          obtained.                                                                          obtained.                                                                          obtained.                                                                          obtained.                          amount of                                                                           amount of                                                                           because                                                           the   the   the                                                               foaming                                                                             foaming                                                                             amount of                                                         raw   raw   the glass                                                         material                                                                            material                                                                            material                                                          was   was   was                                                               insuffi-                                                                            exces-                                                                              large.                                                            cient.                                                                              sive.                                                          __________________________________________________________________________

As shown in Table 1, for the sample No. 1 in which the content ofpearlite added as a foaming material was less than that in the presentinvention, a molten glass coating film having a hollow structure couldnot be obtained because of the low content of the foaming raw material.On the other hand, for the sample No. 2 in which the pearlite added as afoaming raw material was more than in the present invention, a moltenglass coating film was broken because of the excessive content of thefoaming raw material. Accordingly, the content of the foaming rawmaterial is most desirably in a range from 2 to 50 wt %, and,accordingly, roseki added as a refractory powder becomes in the rangefrom 50 to 98 wt %. For the sample No. 3 in which the content ofboro-silicate glass added as a glass component melted at 400°-1500° C.was more than that in the present invention, a molten glass coating filmhaving a continuous hollow structure could not be obtained because ofthe down-flow and coagulation of glass due to the reduction in theviscosity of the molten glass.

On the other hand, in the present invention, the following wasconfirmed: in each of the samples Nos. 4, 5, 6 and 7 using unfoamedpearlite, a continuous molten glass coating film having a uniform hollowstructure over the surface was formed; and in each of the sample Nos. 8and 9 using unformed pearlite and vermiculite previously foamed byheating, a continuous molten glass coating film having a uniform hollowstructure over the surface was also formed.

On the basis of the results of Table 1, a test was performed to confirmthe heat-insulating effect of the present invention using a gas furnace.

FIG. 1 is a vertical sectional view showing the measurement state. Inthis figure, the reference numeral 1 designates a heating box; 2 is analumina-graphite nozzle; 3 is a thermocoupler; and 4 is a burner. Eachof the heat-insulating refractory materials of the samples Nos. 6 and 9shown in Table 1 were formed on the outer surface of thealumina-graphite casting nozzle 2 by brushing, thus forming aheat-insulating layer 5 having a thickness of 5 mm. This was heated at1100° C. and was left in the atmosphere. Thus, the temperature drop ofthe samples from 1100° C. was examined. The sample using theconventional ceramic fibers and the sample without heat-insulatingmaterial were examined as comparative examples. As for the measurementposition, the sample using the ceramic fibers or the sample with foamingheat-insulating material was measured at the interface between theceramic fibers or the foaming heat-insulating material and the castingnozzle.

The results are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                        kind of heat-                                                                 insulating    sample  sample   ceramic                                        material      No. 6   No. 9    fiber  absence                                 thickness     5 mm    5 mm     3 mm   --                                      ______________________________________                                        Temper-                                                                              after 5 min.                                                                             109° C.                                                                         98° C.                                                                       101° C.                                                                       171° C.                        ature  after 10 min.                                                                            194° C.                                                                        180° C.                                                                       183° C.                                                                       369° C.                        drop   after 15 min.                                                                            251° C.                                                                        229° C.                                                                       234° C.                                                                       475° C.                        after                                                                         heating                                                                       ______________________________________                                    

Table 2 shows the temperature drop of each sample heated at 1100° C. Thefoaming heat-insulating material of the present invention providedheat-insulation comparable to that of the existing material with ceramicfibers.

In addition, for the material to form bubbles in molten and softenedglass, there may be used various organic or inorganic materialsgenerating blow-holes by heating, such as fly ash and shirasu describedabove, other than the above pearlite and vermiculite. Moreover, for therefractory powder, there may be used powders of silica stone, chamotte,mullite, alumina, fused silica, zirconia and the like. The samples Nos.10, 11, 12, 13 and 14 were prepared using the combination of these rawmaterials, which were examined for heat-insulating effect in a gasfurnace. As shown in Table 3, these samples were confirmed to havesimilar heat-insulating abilities to those of the sample No. 6.

                  TABLE 3                                                         ______________________________________                                        sample No.                                                                             No. 6   No. 10  No. 11                                                                              No. 12                                                                              No. 13                                                                              No. 14                             ______________________________________                                        pearlite (%)                                                                             30    --      --      30    30    20                               fly ash (%)                                                                            --        30    --    --    --    --                                 shirasu (%)                                                                            --                30  --    --    --                                 vermiculite                                                                            --      --      --    --    --      10                               (%)                                                                           roseki (%)                                                                               70      70      70  --    --      70                               chamotte (%)                                                                           --      --      --      70  --    --                                 fused silica                                                                           --      --      --    --      70  --                                 (%)                                                                           boro-silicate                                                                           +50     +50     +50   +50   +50   +50                               glass (%)                                                                     silica sol (%)                                                                         +120    +120    +120  +120  +120  +140                               temperature                                                                            251° C.                                                                        240° C.                                                                        245° C.                                                                      250° C.                                                                      248° C.                                                                      239° C.                     drop after                                                                    heating (after                                                                15 min since                                                                  left)                                                                         ______________________________________                                    

Since the foaming heat-insulating material of the present inventionforms a molten glass coating film having a hollow structure over thewhole surface, the shielding effect for air is superior to that of theheat-insulating fibers, which can suppress the oxidization of graphitecontaining refractories upon heating. In addition, even in the case ofindependently providing the foaming heat-insulating material of thepresent invention on the side wall of a heating furnace, the heatdissipation to the outside of the furnace is reduced, thus obtaining agood heat-insulating effect.

Table 4 shows the relationship between the added amount of organicfibers and the adhesive rate upon spraying in the case of the additionof vinylon fibers as the organic fibers. Here, the wording "adhesiverate" means "(weight of the foaming heat-insulating material stuck onthe surface of a body to be irradiated)÷(total sprayed weight of thefoaming heat-insulating material)×100".

                  TABLE 4                                                         ______________________________________                                        refractory material                                                                             100    100    100   100  100                                powder for foaming heat-                                                      insulation (%)                                                                silica sol (%)   +80    +80    +80   +80  +80                                 organic fiber (%)                                                                                0      1      3     5    6                                 adhesive rate upon                                                                              70     88     90    93   93                                 spraying (%)                                                                  bending strength of                                                           material (MPa)                                                                after drying (110° C. × 24H)                                                      1.5    1.4    1.3   1.3  0.8                                 after heating (350° C. × 3H)                                                      1.3    1.2    1.1   0.9  0.4                                 ______________________________________                                    

As shown in Table 4, as the content of organic fibers is increased, thebending strength of the material is lowered; while even when the contentof the organic fibers is more than 5 wt %, the adhesive rate is notenhanced. The same is true for the other yarn and pulp.

As a result, organic fibers for enhancing the adhesive rate are bestadded in the range of 5 wt % or less of the amounts of the foamingmaterial, refractory powder, liquid binder and glass powder.

INDUSTRIAL APPLICABILITY

The heat-insulating refractory material of the present invention can beused for the surfaces of refractories of linings of an industrialfurnace, the surfaces of refractories for steel casting or the like inorder to suppress heat dissipation.

What is claimed is:
 1. A heat-insulating refractory materialcomprising:a mixture containing 2-50 wt % of a foaming raw materialselected from the group consisting of shirasu, vermiculite, obsidian,pearlite, pitch stone, expanded shale, and fly ash, said foaming rawbeing foamed by heat at a temperature from 400° to 1500° C. and 50-98 wt% of a refractory powder, and a liquid binder which is added to saidmixture in an amount of 20-250 wt % on the basis of the weight of saidmixture.
 2. A heat-insulating refractory material according to claim 1,wherein a glass powder softened and molten at a temperature from 400° to1500° C. is further added in an amount of 0.1-100 wt % on the basis ofthe weight of said mixture.
 3. A heat-insulating refractory materialaccording to claim 1 or 2, in which organic combustible fibers arefurther added in an amount of 0.01-5 wt % on the basis of the weight ofsaid mixture.